(474f) On Aggregation Phenomena in a Disk-Like Hematite Particle Suspension in a Simple Shear Flow By Means of Brownian Dynamics Method

In addition to the magnetic spherical particles, magnetic particles with a variety of shapes and magnetic properties can be synthesized in the form of rod-like, disk-like and cubic-like particles [1-3]. Magnetic suspensions of these particles may be expected to be developed as a functional fluid that exhibits functional characteristics that will be useful for application in specific situations. Representative possible applications are high-density recording materials [4], optical units [5], actuators and dampers [6] and surface modifying technology [7].

In contrast to ordinary ferromagnetic particles, disk-like hematite particles have a characteristic feature in that it is magnetized in the oblate surface direction, normal to the particle axis direction, and it exhibits a weaker magnetization in comparison with the magnetite particles. Hence, characteristic features of orientational distribution and rheology are expected to be significantly different from those of the ferromagnetic spherical or rod-like particles. This has stimulated the motivation for the present study by means of the Brownian dynamics simulations [8,9] that takes into account the spin Brownian motion about the particle axis in addition to the ordinary rotational Brownian motion about a line normal to the particle axis.

From this background, in the present study, we have investigated aggregation phenomena in a suspension composed of disk-like hematite particles by means of Brownian dynamics simulations. The magnetic moment of the hematite particles lies normal to the particle axis direction, and therefore the present Brownian dynamics method takes into account the spin Brownian motion about the particle axis in addition to the ordinary translational and rotational Brownian motion. These particles are assumed to conduct translational and rotational Brownian motion in a simple shear flow with addition of a uniform magnetic field applied in the direction normal to the shearing plane. First, the validity of the present simulation method has been clarified by comparing with results obtained by the fully-established Monte Carlo method. Then, from performing the simulations, we have investigated the influence of the magnetic particle-field and the particle-particle interactions, shear rate and the volumetric fraction of particles on particle aggregation phenomena. Snapshots of aggregate structures are used for a qualitative discussion, and the cluster size distribution, radial distribution function and the orientational correlation functions of the direction of particle axis and magnetic moment are the focus for a quantitative discussion. The main results are summarized as follows. The significant column-like clusters are formed at a magnetic particle-particle interaction much larger than in the case of a magnetic spherical particle suspension. This is because the rotational Brownian motion has a significant influence on the formation of clusters for a suspension composed of disk-like particles with large aspect ratio. An applied magnetic field is found to decrease the formation of column-like clusters. A shear flow is found not to have a significant influence on the internal structures of clusters, but it does influence the cluster size distribution of the column-like clusters.

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